1. Field of the Invention
The invention relates to a circuit for supplying a load with a direct voltage, in particular, for use in high-voltage power supply units, e.g. in the kW range. Such a power supply unit can, for example, be the power supply unit of a gradient amplifier or a resonance converter for a magnetic resonance tomograph apparatus, however, such a circuit is suitable for all other types of high-performance power supply units.
2. Description of the Prior Art
In order to supply a load with a direct voltage, the principle of rectification with capacitive smoothing is often applied. FIG. 1 shows, for example, a one-way rectifier in which a network alternating voltage U.sub.N is adjacent a diode 10 acting as a rectifier diode. The diode 10 conducts only when the network voltage U.sub.N is across positive than an output voltage U.sub.Z across to a smoothing capacitor 12. If the smoothing capacitor 12 is constructed sufficiently large, the power consumption from the network takes place only within a narrowly limited time on both sides of the maximum of the network voltage U.sub.N. Current peaks thereby occur whose magnitude is in principle limited only by the network impedance. Such current peaks cause undesired harmonics in the network, and lead to a flattening of the network voltage U.sub.N in the region near the voltage maximum.
In order to reduce these undesired effects, the rectifier is conventionally not directly connected to the current network, but rather via a passive filter (not shown in FIG. 1). Such a filter limits and distributes the current consumption from the network. At the power values that are of interest here, a sufficiently dimensioned filter is large, heavy and expensive. In addition, it is expected that the relevant regulations of the electrical utility companies will become more stringent in the future, causing the required expense to increase further.
FIG. 2 shows a known rectifier bridge circuit for a three-phase current network. Three network input terminals 20, 20', 20" are connected respectively with rectifier branches via a filter 22. Each rectifier branch contains two diodes 24, 26; 24', 26'; 24", 26" connected in series with the same polarity. The three rectifier branches are connected in parallel and with a smoothing capacitor 28. Overall, the circuit according to FIG. 2 provides an output voltage U.sub.Z which is always at least as large as the largest difference between each two phases of the three-phase current network. The negative side of the output voltage U.sub.Z is at an output terminal 30, and the positive side of the output voltage U.sub.Z is at to an output terminal 32.
In this three-phase rectifier circuit as well, for the reasons already cited the filter 22 causes a considerable expense.